1. Field of the Invention
The present invention relates to the improvement of a solar cell and a fabrication method thereof.
2. Description of the Background Art
In general, a conventional solar cell has a structure as shown in FIG. 9, for example. The fabrication process is set forth in the following. In the present specification, the light receiving surface or side is referred to as the front surface or side and the opposite surface or side is referred to as the back surface or side.
As shown in FIG. 9A, a single crystalline silicon substrate (wafer) 100 having a thickness of 30-500 .mu.m and including boron of approximately 1.times.10.sup.15 cm.sup.-3 is provided. A polycrystalline silicon substrate may also be used. A p type diffusion layer having high concentration of at least 1.times.10.sup.17 cm.sup.-3 is locally formed at the back surface. This portion becomes the local BSF (Back Surface Field) layer. The fabrication method includes the steps of covering the substrate with a mask material 101 such as a silicon oxide film excluding the region at the back surface where a diffusion layer is to be formed, heating the wafer at a temperature of 500-1200.degree. C. in an atmosphere of the mixture of BBr.sub.3 gas and oxygen to form an oxide including boron of high concentration all over the wafer, and applying solid-phase diffusion to implant boron of high concentration only into the silicon substrate region at the back surface that is not masked to form p type diffusion layers 102 of high concentration.
Then, as shown in FIG. 9B, the oxide film and the mask material are removed from the surface of the wafer. As shown in FIG. 9C, an n type diffusion layer 103 having a high concentration of at least 1.times.10.sup.18 cm.sup.-3 is formed at the front surface. In a similar manner, the back surface is covered with mask material 101. Then, the wafer is heated at the temperature of 500-1200.degree. C. in an atmosphere of the mixture of POCl.sub.3 gas and oxygen, whereby an oxide including phosphorous of high concentration is formed all over the surface of the wafer. By solid-phase diffusion therefrom, phosphorous of high concentration is implanted only into the silicon substrate at the front surface that is not masked. As a result, n type diffusion layer 103 is formed. Thus, an n.sup.+ diffusion layer 103 is formed at the light receiving surface, and p.sup.+ diffusion layers 102 are locally formed at the back surface. After mask material 101 is removed, the surface is entirely covered with a SiO.sub.2 film 104.
As shown in FIG. 9D, an N electrode 105 of Ag/Pd/Ti is provided in a comb-like manner at the portion where unnecessary insulation film is removed. At the back surface, a P electrode 106 of Ag/Pd/Ti/Al is formed at the entire back surface. Thus, a local BSF type single crystalline silicon solar cell is produced. FIG. 9E shows a bottom view of this solar cell. FIG. 9F is a sectional view of the solar cell taken along line X-Y of FIG. 9E. Although not illustrated, an anti-reflection coating is provided at the light receiving surface.
When an n type substrate is used, the p type and n type described above are interchanged.
The structure in which a potential gradient from the front surface to the back surface is incorporated by providing an n.sup.+ /p (or p.sup.+ /n) junction at the front surface of a p.sup.- (or n.sup.-) type substrate and a p.sup.+ /p.sup.- (or n.sup.+ /n.sup.-) low-high junction at the back surface is a general structure for a silicon solar cell having electrodes located at both surfaces.
A solar cell that has electrodes only at the back surface as shown in FIG. 10 is also known. Such a solar cell is fabricated by the following processes.
As shown in FIG. 10A, a single crystalline silicon substrate (wafer) 100 including low concentration boron of approximately 1.times.10.sup.13 cm.sup.-3 with a thickness of 30-500 .mu.m is provided. At the back surface, p type diffusion layers having high concentration of at least 1.times.10.sup.17 cm.sup.-3 that become the BSF layer are formed in an island-like manner.
The fabrication method includes the steps of covering both the front surface and the back surface with a mask material 101 such as a silicon oxide film, removing mask material 101 from predetermined portions at the back surface by photolithography and the like, heating the wafer at the temperature of 500-1200.degree. C. in an atmosphere of the mixture of BBr.sub.3 gas and oxygen, whereby an oxide including boron of high concentration is formed all over the surface of the wafer. By solid-phase diffusion therefrom, boron of high concentration is implanted only into the portion of the back surface that is not masked to form a p type diffusion layer 102. After removing the oxide and mask material from the surface of the wafer as will be described afterwards, n type diffusion layers having high concentration of at least 1.times.10.sup.18 cm.sup.-3 are formed in an island-like manner at the back surface, before an N electrode is formed.
As shown in FIG. 10B, mask material 101 covering both the front and back surfaces is removed from predetermined portions at the back surface. By heating the wafer at the temperature of 500-1200.degree. C. in an atmosphere of the mixture of POCl.sub.3 gas and oxygen, an oxide including phosphorous of high concentration is formed all over the surface of the wafer as shown in FIG. 10C. By solid-phase diffusion, phosphorous of high concentration is implanted only into the portion of the back surface that is not masked. Thus, island-like n.sup.+ diffusion layers 107 and p.sup.+ diffusion layers 102 are provided at the back surface opposite to the light receiving surface.
As shown in FIG. 10D, following removal of unnecessary insulation film, a silicon oxide film 104 is deposited by CVD and the like all over the back surface. A window is formed in the CVD oxide film by photolithography to make contacts with the island-like n.sup.+ diffusion layers and p.sup.+ diffusion layers. Electrodes of Ag/Pd/Ti/Al are provided at the portion where the window is formed. The electrodes are formed in a comb-like manner, divided into an N electrode that connects the island-like n.sup.+ diffusion layers with each other and a P electrode that connects the island-like p.sup.+ diffusion layers with each other. FIG. 10E is the bottom view of the wafer prior to formation of the electrodes. FIG. 10F is a sectional view of the wafer after the electrodes are provided, taken along line x-y of FIG. 10E. Then, an anti-reflection coating is formed at the light receiving surface.
The structure in which a potential gradient is incorporated into the substrate by providing an island-like n.sup.+ /p.sup.- (or p.sup.+ /n.sup.-) junction and a p.sup.+ /p.sup.- (or n.sup.+ /n.sup.-) low-high junction at the back surface of a p.sup.- or n.sup.+ type substrate of low concentration is the general structure of the back surface double electrode contact type solar cell having both electrodes located at one side.
For solar cells which should convert sunlight of low energy density into power, one continuing goal is to improve the photoelectric conversion efficiency thereof. From the standpoint of effective usage of incident energy, there is a problem that long wavelength light is not utilized and lost, though light of the relatively short wavelength portion of the solar energy is substantially entirely converted photoelectrically at the proximity of the light receiving surface of the solar cell.
As an approach of utilizing light of the long wavelength portion, there may be a structure of altering the band gap of the substrate within the solar cell to allow photoelectric conversion lower energy of the long wavelength light.
However, it is generally difficult to alter the band gap within the substrate in a solar cell employing a single crystal based substrate. This is because the band gap in the single crystal having a uniform physical property is essentially uniform everywhere. For the alteration of the band gap, one semiconductor material may be superposed on other semiconductor materials, but there are few such other semiconductor materials that can match in lattice constant and thermal expansion coefficient.
An amorphous semiconductor that can have the physical constant altered rather arbitrarily in the direction of the film thickness includes in the whole film many factors that become the core of minority carrier recombination such as the intra-gap level and grain boundary. In such an amorphous material, the diffusion length of the carrier that greatly affects the conversion efficiency of the solar cell is relatively shorter than that in a single crystal. Therefore, it is expected that high efficiency cannot be achieved in the amorphous material without an epoch-making idea. In conventional art, the film deposition technique for semiconductor materials to have different band gaps is critical. The film is usually grown using CVD. In this case, the choice of material gases and setting the film growing conditions are difficult factors to manage. Furthermore, film formation is sensitive to the state of the substrate. It can be said that growing a film per se has various problems. The film could be grown on the entire processed surface rather than be restricted in a desired region.